LASER IONIZATION MEASUREMENTS OF GASEOUS GALLIUM ATOM DESORPTION FROMGAAS(100) DURING MOLECULAR-BEAM EPITAXY

The desorbed fluxes of gallium atoms from a GaAs(100) substrate during GaAs growth in the temperature range 550 to 1015 K are directly measu red with laser single-photon ionization time-of-flight mass spectromet ry (SPI-TOFMS). At substrate temperatures above 900 K, Ga atoms desorb from the bulk GaAs wafer. The activation energy for this desorption u nder neither an incident As nor Ga flux is 4.7 +/- 0.2 eV. Under an in cident As-4 flux, the measured activation energy for Ga desorption fro m the bulk is 5.2 +/- 0.2 eV. At substrate temperatures between 600 K and 900 K, scattered gallium atoms are detected during GaAs homoepitax y with incident As-4 and Ga fluxes. The flux of Ga atoms increases wit h increasing GaAs surface temperature, but no flux is observed when th e Ga oven shutter is closed. Simultaneous measurements of reflection h igh-energy electron diffraction (RHEED) specular beam intensity oscill ations and scattered As-2 and As-4 mass signals are recorded. The stic king probability of Ga atoms during GaAs homoepitaxy derived from the RHEED results is found to decrease from unity to 0.92 +/- 0.02 with in creasing temperature. The desorbing flux of Ga atoms linearly increase s with incident Ga flux and is not appreciably affected by changes in the incident As-4 flux. A precursor mediated adsorption model for Ga a toms on GaAs(100) is discussed to interpret the results. (C) 1997 Else vier Science B.V.